This invention relates to coupling of polyolefins, more specifically coupling of polyolefins using insertion into carbon hydrogen (Cxe2x80x94H) bonds.
As used herein, the term xe2x80x9crheology modificationxe2x80x9d means change in melt viscosity of a polymer as determined by dynamic mechanical spectroscopy. Preferably the melt strength increases while maintaining the high shear viscosity (that is viscosity measured at a shear of 100 rad/sec by DMS) so that a polymer exhibits more resistance to stretching during elongation of molten polymer at low shear conditions (that is viscosity measured at a shear of 0.1 rad/sec by DMS) and does not sacrifice the output at high shear conditions.
Polyolefins are frequently rheology modified using nonselective chemistries involving free radicals generated for instance using peroxides or high energy radiation. However, chemistries involving free radical generation at elevated temperatures also degrade the molecular weight, especially in polymers containing tertiary hydrogen such as polystyrene, polypropylene, polyethylene copolymers etc. The reaction of polypropylene with peroxides and pentaerythritol triacrylate is reported by Wang et al., in Journal of Applied Polymer Science, Vol. 61, 1395-1404 (1996). They teach that branching of isotactic polypropylene can be realized by free radical grafting of di- and tri-vinyl compounds onto polypropylene. However, this approach does not work well in actual practice as the higher rate of chain scission tends to dominate the limited amount of chain coupling that takes place. This occurs because chain scission is an intra-molecular process following first order kinetics, while branching is an inter-molecular process with kinetics that are minimally second order. Chain scission results in lower molecular weight and higher melt flow rate than would be observed were the chain coupling not accompanied by scission. Because scission is not uniform, molecular weight distribution increases as lower molecular weight polymer chains referred to in the art as xe2x80x9ctailsxe2x80x9d are formed.
The teachings of U.S. Pat. No. 3,058,944; 3,336,268; and 3,530,108 include the reaction of certain poly(sulfonyl azide) compounds with isotactic polypropylene or other polyolefins by nitrene insertion into Cxe2x80x94H bonds. The product reported in U.S. Pat. No. 3,058,944 is crosslinked. The product reported in U.S. Pat. No. 3,530,108 is foamed and cured with cycloalkane-di(sulfonyl azide) of a given formula. In U.S. Pat. No. 3,336,268 the resulting reaction products are referred to as xe2x80x9cbridged polymersxe2x80x9d because polymer chains are xe2x80x9cbridgedxe2x80x9d with sulfonamide bridges. The disclosed process includes a mixing step such as milling or mixing of the poly(sulfonyl azide) and polymer in solution or dispersion then a heating step where the temperature is sufficient to decompose the poly(sulfonyl azide) (100xc2x0 C. to 225xc2x0 depending on the azide decomposition temperature). The starting polypropylene polymer for the claimed process has a molecular weight of at least about 275,000. Blends taught in U.S. Pat. No. 3,336,268 have up to about 25 percent ethylene propylene elastomer.
U.S. Pat. No. 3,631,182 taught the use of azido formate for crosslinking polyolefins. U.S. Pat. No. 3,341,418 taught the use of sulfonyl azide and azidoformate compounds to crosslink certain thermoplastics (PP (polypropylene), PS (polystyrene), PVC (poly(vinyl chloride)) and their blends with certain rubbers(polyisobutene, EPDM,(ethylene propylene diene rubber)).
Similarly, the teachings of Canadian patent 797,917 (family member of NL 6,503,188) include rheology modification using from about 0.001 to 0.075 weight percent poly(sulfonyl azide) to modify homopolymer polyethylene and its blend with polyisobutylene. It would be desirable to have polymers rheology modified rather than crosslinked (that is having less than about 10 percent gel as determined by xylene extraction specifically by ASTM 2765). Which polymers are advantageously made using single site catalysts, preferably single site metallocene or single site constrained geometry catalyst and, thus, desirably of narrow molecular weight distribution (MWD=Mw/Mn, where Mw is the weight average molecular weight, and Mn is the number average molecular weight) (that is having MWD preferably less than or equal to about 3.0, most preferably less than about 2.5 Mw/Mn). The surprising results are especially evident when the starting material is high density polyethylene (density greater than 0.945 g/ml)(hereinafter HDPE of narrow MWD) which polymers advantageously have a combination of good processibility as indicated by higher melt strength at a constant low shear viscosity e.g. 0.1 rad/sec measured by DMS, and higher toughness, tensile and/or elongation than a high density polyethylene of broader molecular weight distribution treated with sulfonyl azides according to the practice of the prior art using the same equivalents (stoichiometry) of coupling reactant to polymer, and higher toughness than that of the same starting material coupled or rheology modified using the same equivalents of a free radical coupling agent. Desirably, the product would have better organoleptics than coupled broader MWD HDPE. Advantageously, compositions would have less undesirable odor than the same starting materials coupled or rheology modified using the same chemical equivalents of free radical generating agents. Preferably, a process of the invention would result in more consistent coupling than methods of coupling involving free radicals, that is use of the same reactants, amounts and conditions would result in consistent amounts of coupling or consistent (reproducible) property changes, especially consistent amounts of gel formation. Preferably, a process would be less subject to effects from the presence of oxygen than would a coupling or rheology modification involving agents which generate free radicals.
In the case of, medium and lower density polyethylene (that is polymers having a density of from about 0.94 g/cc to about 0.90 g/cc), which are advantageously copolymers of ethylene in which the percent comonomer is preferably about 0.5 to 5 mole percent comonomer based on total polymer as determined by ASTM 5017, the polymers would desirably show a combination of processability improved over the starting material with retention of toughness, low heat seal initiation temperature, low haze, high gloss or hot tack properties characteristic of the starting material.
In the case of elastomeric polymers containing ethylene repeating units in which the preferred comonomer content is about 5-25 mole percent, and preferably a density less than about 0.89 g/mL, it would be desirable to have better mechanical properties such as elongation and tensile strength than would be achieved in the starting material or by coupling using the same chemical equivalents of free radical generating agent like a peroxide.
It has now been found that polymers having a narrow molecular weight distribution or formed using single site catalysts, especially polyolefins formed using transition metal catalysts other than Ziegler Natta catalysts, particularly where the molecular weight distribution is 3 or less, are surprisingly effectively coupled using poly(sulfonyl azide) coupling agents. The resulting coupled polymers have at least one of the desirable properties listed.
The invention includes a process of preparing a coupled polymer comprising heating an admixture containing (1) at least one polyolefin comprising ethylene and optionally at least one comonomer which is selected from alpha olefins having at least 3 carbon atoms, dienes and combinations thereof said polyolefin having a molecular weight distribution of less than or equal to about 3 and (2) a coupling amount at least one poly(sulfonyl azide) to at least the decomposition temperature of the poly(sulfonyl azide) for a period sufficient for decomposition of at least about 80 weight percent of the poly(sulfonyl azide) and sufficient to result in a coupled polymer; particularly where the polyolefin is the product of polymerization of ethylene and optionally at least one other alpha-olefin in the presence of a single site catalyst (e.g. transition metal like V or Cr, metallocene or constrained geometry). The amount of poly(sulfonyl azide) is preferably from about 0.01 to about 5 weight percent of the polyolefin. The poly(sulfonyl azide)and polyolefin preferably react at a temperature at least the decomposition temperature and greater than about 150xc2x0 C. The invention also includes any composition made by the process of the invention and any article made from the composition as well as processes for making the articles, particularly melt processing (especially where the article is formed from a melt of a composition of the invention), more preferably selected by molding, blow molding or extrusion blow molding the composition. The invention also includes the use of a composition of the invention in any of these processes.